1. Technical Field
The presently disclosed subject matter relates to a surface mount light emitting apparatus, and in particular, to a surface mount LED apparatus in which an LED chip mounted on a substrate is sealed with a resin material.
2. Description of the Related Art
A surface mount LED apparatus, such as one disclosed in Japanese Patent Application Laid-Open No. 2006-190888 (corresponding to U.S. Patent Publication No. 2006/0151799A1), has a pair of patterned circuits which are formed from the top surface of the substrate to the bottom surface via the respective side surfaces of the substrate. An LED chip is mounted on the top surface of the substrate. The LED chip has a bottom electrode which is electrically connected to one of the pair of patterned circuits via an electrically conductive adhesive or the like. The LED chip also has a top electrode which is electrically connected to the other of the patterned circuits by wire bonding or the like. The LED chip, together with the bonding wire and part of the patterned circuits, is sealed with a resin material which is transparent to the wavelength of emitted light.
Such a surface mount LED apparatus can be mounted on a component substrate together with other electronic components. For this purpose, reflow soldering is typically employed. During reflow soldering, the entire assembly is heated to a high temperature, such that the difference in thermal expansion coefficient between the sealing resin material of the surface mount LED apparatus and the patterned circuits typically causes thermal stress on the interface between the patterned circuits and the sealing resin material. This thermal stress may in turn act to lift the LED chip above the patterned circuits, thereby causing the LED chip to be separated from the patterned circuits resulting in faulty electrical continuity therebetween.
To prevent this electrical discontinuity, a technique described in Japanese Patent Application Laid-Open No. 2006-190888 is employed in which a portion underneath the LED chip has no patterned circuit formed thereon, so that an insulating substance which constitutes the substrate is exposed. Mounting the LED chip on the patterned circuits provides a gap corresponding to the height of the patterned circuits between the exposed insulating substance and the bottom surface of the LED chip, so that the sealing resin goes into this gap to bring the substrate and the sealing resin into tight contact therebetween. The adhesion between the sealing resin material and the insulating substance (the substrate) is greater than that between the resin material and the metal material (forming the patterned circuits). This prevents the LED chip from being separated from the patterned circuits even in the presence of thermal stress, which is caused by heating during reflow soldering, between the sealing resin material and the patterned circuit.
Although the technique described in Japanese Patent Application Laid-Open No. 2006-190888 provides an improvement in the adhesion between the LED chip and the substrate, the thermal stress during reflow soldering is also produced on the interface between the sealing resin material and the LED chip. This may lead to interfacial separation by which the sealing resin material and the LED chip are separated from the interface therebetween.
An occurrence of separation between the sealing resin material and the LED chip on the interface therebetween would result in a microscopic void appearing at the separated portion, thereby causing deterioration in the intensity of light emitted from the LED chip. Since the LED chip is typically cubic in shape, the stress on the interface between the chip and the surrounding resin material tends to concentrate on its corner, thereby causing separation from the corner. Once separation occurs at a particular portion, the adhesion of that portion is reduced thereby causing the stress to be relieved at the separated portion and the area of separation then can expand.